It is known to prepare thermoplastic blends of cross-linked rubbery polymers such as butyl rubber and polypropylene, useful whenever a high tensile strength semi-rigid plastic is required, by dynamic cure of the blends, Gessler et al, U.S. Pat. No. 3,037,954. The rubbery polymer is about 5 to 50% of the total of rubbery polymer and polypropylene. Dynamic curing is described as a process which initially forms a cross-linked or three dimensional structure which, thereafter, is broken down and dispersed in the uncross-linked matrix polymer as a micro gel.
It is also known that blends of elastomeric and alkene plastomeric polymers can be prepared by mixing together in an internal mixer 100 parts by weight of polyethylene and 5 to 70 parts by weight, preferably 5-50 parts by weight of an elastomeric polymer such as butyl rubber, a free radical type catalyst and a bifunctional aromatic compound to produce blends useful in the production of harder, tougher and more durable burst resistant rigid pipes, Fischer U.S. Pat. No. 3,073,797.
Similarly, it is known to blend from about 60%-85% by weight of an ethylene copolymer, having up to about 5% of an alpha olefin copolymerized with the ethylene with 15 to 40% by weight of butyl rubber to provide a composition which is easily cured or vulcanized either prior to the formation of a shaped article such as tubular film or simultaneously with the formation of the shaped article, Zagar U.S. Pat. No. 3,184,522.
More recently, extrudable thermoplastic modified olefin polymers have been described which are prepared by mixing and heating at least about 40 parts by weight of an unsaturated olefin polymer up to 60 parts by weight of an unsaturated uncured synthetic rubber, such as butyl rubber, and a minor amount of a bifunctional phenolic material, Hartman U.S. Pat. No. 3,909,463. The conditions were designed to avoid cross-linking or vulcanization and to prepare grafted block copolymers, instead. The copolymers contain the polyolefin in an amount of at least 50% by weight.
The preparation of elastomeric composition, i.e., compositions having predominantly elastomeric character of butyl rubber have heretofore eluded the art because, it is believed, of the mistaken belief that mixing and heating a butyl rubber-polyolefin blend having a high proportion of butyl rubber with curing agents would necessarily give a "scorched" granulated thermoset. See in this connection, Hartman supra and Fischer U.S. Pat. No. 3,806,558, which with reference to EPDM rubber and polyolefin, teaches that the rubber must be only partially cured to attain fabricability.